Maybe it doesnt work in the toys, and you need to get them separately, but it could work in the movie. They didnt need to make two sets of constructicons to do that. And in any case, it seemed like Devastator wasnt formed by robots, but by vehicles. Like the vehicles werent constructicons but they only formed Dev (like they said the bikes would do for Arcee).
In any case, Bay could have used Barricade or other robots if the constructicons were busy forming Dev.

Finally, i find it stupid that Demolishor and that constucticon are 99% identical. It seems to me that Bay looked at him and went
"gee, i could use this guy to tear up Shanghai and make a great opening scene".
"But michael, what about Devastator?"
"Just make up another identical robot!"